SmedSpeed XS650


rocker arms and cam timing

When the Yamaha XS650 left the factory, the rocker arms were manufactured with a precisely ground radius on the area that contacts the cam and this was then hard chrome plated. Over many years the hard chrome wears, mostly due to extended oil changes, oil with no ZDDP in it, bad filtration and just general wear and tear (abuse). This wear has an adverse effect on the cam timing. Pictured below is a rocker arm I took out of a running engine. It did sound like a bag of bolts, but it was running nevertheless. This is the worst example I have seen, but many rocker arms which look good are not within specification, and the accumulated wear will only get worse over time.

Worn rocker arm

As part of my engine rebuilds, I dial in the cam timing using a degree wheel and a dial gauge mounted on the valve spring top collar. This way the cam timing can be set very accurately, and also any manufacturing discrepancy in the cam or the rocker arms can be traced. The performance and rephased cams I supply are ground on a Landis 3L cam grinding machine, many of the USA based cam manufacturers use this same equipment, it is state of the art, so I am entirely confident the cams are not the variable.

Landis 3L
I often find that my cam timing figures are not what I expect, sometimes way off what they should be, and this is invariably attributed to worn rocker arms. The chrome plating wears away and the radius "flattens" or small gouges are evident, all of which affect the valve opening duration, the cam lift strangely is essentially unaffected. To remedy the situation I have found a firm who can recondition the OEM rocker arms by replacing the radiused part of the rocker arm with a brazen on radiused stellite pad. This technology has been used to reclaim many thousands of Ducati rocker arms which also suffer the same fate as the XS650 items. The performance cams I provide are made from EN40B steel and then case hardened. The Stellite/EN40B is an ideal combination for a long lasting hard wearing valve gear.

Reconditioned rocker arm

So why bother at all I hear you say? There are many reasons for doing something properly, not least just to get it right. If you are fitting a new cam, whether it is for more performance or as part of a rephased engine build, it seems pointless not to do it the best it can be.

The engines I build are not in the main dedicated racing engines, just strongly built high torque engines. This increased torque allowed the bike to run higher gearing to cope with increased traffic speeds. Worn rocker arms translate the duration of what was a very mild cam in the first place into something more suitable for a lawn mower. A good mild street cam coupled with refaced rocker arms will really bring your engine to life.

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These are experimental parts intended for racing and as such no warranty is given or implied. If you decide to buy it is on the understanding that you assume complete responsibility for any possible injury, loss or damage that may occur as a result of using these parts and that you accept these conditions of sale prior to purchase.